The acid value is the number which expresses in milligrams the amount of potassium hydroxide necessary to neutralize the free acids present in 1 g of the substance.

Method

Unless otherwise specified in the individual monograph, dissolve about 10 g of the substance under examination, accurately weighed, in 50 ml of a mixture of equal volumes of ethanol (95 per cent) and ether, previously neutralized with 0.1 M potassium hydroxide to phenolphthalein solution. If the sample does not dissolve in the cold solvent, connect the flask with a reflux condenser and warm slowly, with frequent shaking, until the sample dissolves. Add 1 ml of phenolphthalein solution and titrate with 0.1 M potassium hydroxide until the solution remains faintly pink after shaking for 30 seconds. Calculate the acid value from the expressionAcid value = [5.6] n/wWhere, n = the number of ml of 0.1 M potassium hydroxide required;w = the weight, in g, of the substance.NOTE - If the oil has been saturated with carbon dioxide for the purpose of preservation, gently reflux the solution of the oil in ethanol (95 per cent) and ether for 10 minutes before titration. The oil may be freed from the carbon dioxide by exposing it in a shallow dish in a vacuum desiccator for 24 hours before weighing the sample.